In a wavelength division multiplexing (WDM) optical transmission system, optical signals at a plurality of wavelengths are encoded with digital streams of information. These encoded optical signals, or “wavelength channels”, are combined together and transmitted through a series of spans of optical fiber in a WDM fiberoptic network. At a receiver end of a transmission link, the wavelength channels can be separated, whereby each wavelength channel is individually detected by an optical receiver.
While propagating through an optical fiber, light gets attenuated via absorption and scattering. Yet some minimal level of optical power is required at the receiver end to decode information that has been encoded in a wavelength channel at the transmitter end. To boost optical signals propagating in an optical fiber, optical amplifiers are deployed at multiple locations, known as nodes, throughout the transmission link. Optical amplifiers extend the maximum possible length of the link, in some instances, from a few hundred kilometers to several thousand kilometers, by amplifying optical signals to power levels close to the original levels of optical power at the transmitter end.
Even when amplified to original power levels, WDM optical signals cannot be restored to their original condition, because optical amplifiers add in-band noise to the signal. The optical noise effectively limits a maximum length of the transmission link, and therefore needs to be tightly controlled and measured.
One straightforward method to measure in-band signal-to-noise ratio (SNR) is to convert optical signal to electrical signal and to measure the SNR by demodulating the electrical signal. However, the straightforward SNR measurement requires costly electronic equipment. Thus, measuring SNR in optical domain (so-called “optical SNR” or OSNR) is highly desirable.
OSNR can be evaluated by scanning the spectrum of the WDM optical signal, detecting wavelength channels which reveal themselves as spectral peaks in the WDM optical signal, and evaluating ratio of peaks to valleys in the spectrum. This method, however, is inherently imprecise, because it assumes that optical noise in the valleys between the wavelength channels is the same as in the in-band optical noise. It is not uncommon that in-band noise is actually much higher than out-of-band noise.
A more precise method relies on a difference between polarization properties of wavelength channels and optical noise. The WDM signal light is generated by laser diodes, which emit polarized light. Upon optical amplification, the light polarization is generally preserved. As a result, a degree of polarization of the wavelength channels is high (when polarization mode dispersion (PMD) is low enough). On the other hand, optical noise remains unpolarized. This is because optical noise in optical fiber amplifiers originates from randomly polarized spontaneous emission of light in optically inversed gain medium, which is amplified by the same gain medium that emitted it. A high degree of polarization of the useful optical signal and lack of polarization in the optical noise allows one to suppress wavelength channels one by one using polarization nulling methods, and directly measure the remaining optical noise. The measurement is repeated for each wavelength channel of interest.
Chung et al. in US Patent Application Publication 2004/0114923 disclose an OSNR monitoring system including a polarization controller coupled to a linear polarizer and a tunable optical bandpass filter. The tunable optical bandpass filter is tuned to a wavelength channel of interest. Since the polarization state of the wavelength channels is not known, the polarization controller scans the polarization within a predetermined range, and a minimum value is searched for. When the polarization direction of the optical signal at the output of the polarization controller is orthogonal to the polarization transmission direction of the polarizer, the transmitted optical power is at minimum, being equal to one half of the optical noise power. Once the optical noise power is known, the OSNR can be calculated.
Yao in U.S. Pat. Nos. 7,218,436; 7,391,977; and 8,000,610 discloses a system for measuring OSNR by either scrambling polarization of WDM optical signal, or by systematically varying through all possible states of polarization, and detecting maximum and minimum optical power levels at a photodetector disposed downstream of an optical polarizer. The system of Yao also includes a tunable optical filter for selecting individual wavelength channels of the WDM optical signal.
Detrimentally, the OSNR measuring systems of Chung and Yao rely on scanning a polarization controller through all polarization states to find a particular setting of the polarization controller, at which the optical signal from a particular wavelength channel is suppressed. Due to a great multitude of possible polarization states of a polarization controller, such scanning can take an impractically long time. Polarization scrambling, that is, quickly and randomly changing polarization of the WDM optical signal, can be used in an attempt to shorten the scanning time at each wavelength. However, polarization scrambling does not guarantee that the required polarization state is always achieved, thus reducing fidelity of OSNR measurements.
Chung et al. in U.S. Pat. No. 7,257,324 disclose an OSNR monitoring apparatus including a polarization controller coupled to a polarization-selective optical delay line, for imparting a controllable amount of a differential group delay (DGD) to the modulated optical signal. A fast photodetector is coupled to the polarization-selective optical delay line for measuring DC and AC components of the modulated optical signal. At a certain pre-defined amount of DGD imparted to the optical signal, the DC component becomes proportional to a magnitude of the wavelength channel signal, while the AC component is proportional to the optical noise. Thus, by measuring ratio of DC electrical signal to AC electrical signal at the photodetector output, OSNR can be estimated.
Detrimentally, the apparatus of Chung et al. in U.S. Pat. No. 7,257,324 requires rather complex electronics for processing high-frequency electrical signals. Furthermore, the optimal delay has to be found in advance before proper calculations can be carried out, the signal has to be stable in time, and non-linear effects must not degrade the spectral characteristic of the signal to be measured.